In French application No. 76 39723 filed Dec. 31, 1976, a device controlled by the inclination of the head of a driver and preventing his falling asleep has been described. The device comprises essentially a detector of the changes of the head positions and an audible warning. The embodiment shows a detector placed on the ear and an alarm positioned in a relatively large box. This device, in use, has proven itself impractical, fragile and of insufficient sensitivity. For example, the shape of the envelope of the detector-interrupter requires precise positioning on the ear and its necessary displacement for establishing (or breaking) the circuit of the alarm renders it troublesome to the user who has little tolerance for it.